The research in this project is focused on 1) defining the role of transforming growth factor-beta1 (TGF-beta1) in the early stages of hepatocarcinogenesis, and 2) isolation and characterization of a novel liver growth inhibitor (LDGI). We have used the spontaneous transformation of rat liver derived epithelial (RLE) cells as a model to examine the change in the growth inhibitory effects of TGF-beta1 during the transformation process. The appearance of morphologically aberrant transformants correlated directly with an increased resistance of the population to the growth inhibitory effects of TGF-beta1. Clonal cell lines derived from the transformants were resistant to TGF-beta1-dependent inhibition of DNA synthesis. These cell lines were also highly tumorigenic, aneuploid with characteristic gross chromosomal abnormalities, and expressed a number of phenotypic markers common to rat liver epithelial cells transformed by oncogenes or chemicals. In contrast, apparently normal-looking cell lines cloned from the same population were non-tumorigenic, near-diploid with few chromosomal gross abnormalities and were as sensitive to TGF-beta1 as early passage normal RLE cells. Morphologically normal late passage rat liver epithelial cells were sensitive to transformation by the DNA hypomethylating agent 5-aza-2-deoxycytidine, in contrast to earlier passage cells, and this transformation was accompanied by the development of resistance to the growth inhibitory effects of TGF- beta1. These findings suggest that acquisition of resistance to the effects of growth inhibitors such as TGF-beta1 is an important and possibly essential stage in the spontaneous transformation of RLE cells. In addition to the TGF-beta1 studies we are presently attempting to obtain partial protein sequence data on the highly purified fraction of LDGI.